This invention relates generally to systems for examining brain wave responses to stimulation of a subject, and more particularly to a system wherein evoked potentials responsive to selected critical verbal stimuli are analyzed mathematically to establish a probabilistic value corresponding to the information content of the evoked signal responses.
It has long been proposed that the unconscious mind is psychologically meaningful in that there exists a psychic continuity whereby seemingly discontinuous psychological patterns are actually continuous, but certain psychological events are unknown and in the unconscious. It has therefore been reasoned that the existence of an unconscious must be assumed in psychoanalysis, but methods other than the clinical methods of psychoanalysis must be employed to demonstrate the unconscious. The experimental stimulus used in some experimental efforts to demonstrate the unconscious is a picture of a pen pointing at a knee. By tracing the conceptual associations of pen and knee, by words such as "ink," "paper," "foot," and "leg," rational, secondary process thinking was sampled. However, if clang associations were to be traced, then the experiment would be sampling primary process ideation. Examples of such clangs would be "pennant pennant," "happen," "neither," and "any." Finally, two clangs can combine or condense to form a new word, "penny,"which is entirely unrelated in meaning to its components. Associations to this clang condensation can be traced in the form of words such as "coin," "nickel, " "Lincoln," etc. The penny combination is another level of primary-process ideation based on the fact that the stimulus is a pictorial representation of a word, or a rebus, one of the oldest forms of writing and closely allied to dream thinking. Aside from the theoretical relevance of the stimulus, it has the technical advantage of involving no clinical judgment in scoring. Lists of associations based on normative data can be used by assistants with an error rate which is consistently less than 3%.
The rebus method of analysis has been used successfully in various studies. For example, in one experiment, it was shown that pen and knee clang associations and penny rebus associations appeared more often in associations following Stage I, rapid eye movement awakenings, than after Stage II awakenings. On the other hand, pen and knee conceptual associations appeared more frequently following Stage II awakenings than after Stage I, rapid eye movement awakenings. Primary process thought was prominent following dream arousals and secondary-process thought was prominent following one type of non-dream arousal.
It was not, however, until the rebus method was combined with the method of average evoked responses that it became possible to detect directly brain responses to subliminal stimuli and to discover the usefulness of these waves as indicators of complex dynamic and cognitive processes. The average evoked response is based upon the sampling of short periods of the EEG immediately following a given stiumlus. Ordinarily, it is difficult to detect a specific stimulus-locked response in the EEG beacause the EEG reflects so many other simultaneous responses to internal and external stimuli. However, by repeatedly sampling the EEG, a pattern emerges which is directly related to a selected stimulus. It has now been shown that EEG amplitudes within the first 300 milliseconds after stimulation are associated with attention.
In one well known experiment, it was postulated that attentional and perceptual processes were subliminal. This could be tested by presenting two matched stimuli, one of which could be more interesting than the other, and to predict that the more interesting stimulus would elicit a larger brain response. Thus, a matched pair of stimuli were presented in a series of experiments. The experimental stimulus was the fountain pen pointing at a knee, while a controlled stimulus, which matches the experimental one in size, configuration, color, and contour, lacks conventional meaning. FIG. 1A is the experimental stimulus and is a picture of a fountain pen pointed at a leg which is prominently flexed at the knee. FIG. 1B is the control stimulus which is made up of two nonsense figures matching the experimental stimulus in configuration, shape, color, and contour. It was found that one millisecond of exposure of the stimuli to a subject resulted in consistent discrimination between the two stimuli in favor of the rebus. Such discrimination took the form of a larger amplitude in the brain wave with a latentcy of approximately 170 milliseconds.
FIG. 2 is a schematic representation of an averaging method of an EEG signal for the same time epoch. As shown, each response appears different from the other. However, if the different segments are added algebraically, then a consistency emerges reflected in a sizable amplitude. The average evoked response curve shows the appearance of this amplitude. This curve is a total algebraic sum for the amplitude increment, which is then divided by the total number of responses to give the averages.
It therefore has been established by experimentation that a brain wave in the form of an average evoked response discriminates between two subliminal stimuli. Such discrimination is attributable to an amplitude component associated with attention which occurs at approximately between 140 and 80 milliseconds post-stimulus; less than a quarter of a second. Associations to the subliminal rebus stimulus are activated and can be elicited by a free association method. Such free association confirms that thought processes are activated by a subliminal stimulus and persist unconsciously. During such association, the subject is totally unaware of associating more of one category of words than another. The conceptual, secondary-process associations, such as the knee associations, are positively correlated with the size of the discriminating average evoked response amplitude. In other words, the larger the average evoked response amplitude to the rebus stimulation, the more frequently will conceptual secondary-process associations be elicited. This relationship establishes a link between a truly neurophysiological event and an unconscious thought process, for the subjects can in no way be aware of this relationship. However, primary-process associations (clang and rebus words) are not correlated with this amplitude component. Rather, the incidence of primary-process associations is contingent upon the appearance of bursts of rhythmic activity in the alpha range.
Repressiveness, as rated independently on the Rorschach test, is negatively correlated with the magnitude of the discriminating amplitude for the subliminal stimuli. Thus, the more repressive the person is judged on the Rorschach, the smaller will be the average evoked response amplitude in response to the subliminal rebus stimulus. However, when the same stimulus is supraliminal, there is a tendency for the highly repressive person to respond with a larger amplitude. Thus, the highly repressive person responds differently to the same stimulus depending upon whether or not it is subliminal or supraliminal. Additionally, the more repressive the person is, the fewer stimulus-related associations, of all kinds, primary and secondary process, the person will use in free association.
It is a problem with the known methods of analyzing and interpreting average evoked potentials that the information is obtained generally visually by determining the amplitudes of the peaks and their temporal locations in the post-stimulus epoch. There is a need for a system of analysis wherein greater amounts of information can be extracted from the evoked potentials.